driver(9): New devsw members d_cfdriver, d_devtounit.

If set, then bdev_open/cdev_open will use d_devtounit to map the
dev_t to an autoconf instance (e.g., /dev/wd0a -> wd0) and hold a
reference with device_lookup_acquire across the call to d_open.

This guarantees that the autoconf instance cannot be detached while
the devsw's d_open function is trying to open it (and also that the
autoconf instance has finished *_attach before anyone can open it).

Of course, if the underlying hardware has gone away, there will be
I/O errors, but this avoids software synchronization bugs between
open and detach for drivers that opt into it.  It's up to the driver
and bus to figure out how to deal with I/O errors from operations on
hardware that has gone away while the software hasn't finished
notifying everything that it's gone yet.

XXX kernel ABI change to struct bdevsw/cdevsw requires bump

(riastradh)